1. Field of the Invention
The present invention is in the field of helicopters and more particularly relates to an antitorque system replacing the conventional tail rotor.
2. The Prior Art
The advantages of eliminating the conventional tail rotor of a helicopter have long been appreciated. The foremost advantage is the increased safety, particularly to ground personnel. The tail rotor also presents reliability and maintainability problems due to the requirements for multiple right-angle gear boxes, bearings and high rotational speed shafting. In military helicopters the tail rotor is ballistically vulnerable in that a direct hit on the tail rotor can cause sudden and complete loss of antitorque force and yaw control with catastrophic consequences.
The present invention is intended as a replacement for the tail rotor, and its operation is based on the principle of circulation control. Circulation control is a subtle aerodynamic phenomenon in which the bulk flow around a body is deflected by a sheet of air ejected tangentially to the body surface. The deflection of the bulk flow causes a body force in the direction opposite the deflection. Successful force generation by circulation control depends upon many parameters including the strength of the tangential jet relative to the bulk flow, its location and direction, and its thickness. The phenomenon is similar to that by which a baseball is made to follow a curved trajectory.
In U.S. Pat. No. 3,059,877 issued Oct. 23, 1962 to Lee, there is shown a helicopter antitorque system employing the principle of circulation control. That patent describes how exhaust from the main power plant can be conducted rearward through a duct to a chamber in the aft section of the fuselage, from which it is discharged through longitudinally-extending slots located in the downwash of the main rotor.
It is well known in the art that at high speeds of flight the downwash of the main rotor does not flow around the aft section of the fuselage. The fuselage moves out of the way before the downwash can reach it, and, relative to the fuselage, the downwash has a large horizontal velocity component. Thus, the circulation control technique cannot be depended on to provide antitorque forces at high speeds of flight.
This limitation of the circulation control technique can be overcome to some degree by the provision of additional means for producing antitorque forces at high flight speeds. Such means could conceivably include aerodynamic trim surfaces or reaction jets.
A jet of air emerging from the tail of the helicopter in a lateral direction has long been considered as a viable approach to production of antitorque forces. This approach is exemplified in the following U.S. Pat. Nos.: 2,433,251 to Whiting (1947); 2,486,272 to Gazda (1949); 2,503,172 to Pullin (1950); 2,518,697 to Lee (1950); 3,036,068 to Spearman (1962); and 3,510,087 to Strickland (1970).
A helicopter antitorque system combining circulation control with a reaction jet is disclosed in U.S. Pat. No. 3,807,662, issued Apr. 20, 1974 to Velazquez. Velazquez employs a main rotor-driven axial compressor fan and diffuser to supply high velocity air to a variable-geometry nozzle at the end of an enlarged duct extending through the tail boom. An echelon of turnable vanes located within the tail boom at its end permits the generation of either an antitorque force or a forward propulsion thrust, or both, selectively. In addition to this well-known air jet technique, Velazquez implements the circulation control principle by providing circulatipn slots extending lengthwise along the tail boom and positioned within the maximum velocity region of the downwash from the main rotor blades. Some of the high pressure air generated by the compressor fan is discharged through the slots tangentially to the surface of the tail boom to provide an antitorque moment after the manner of Lee, supra. The slots are provided with automatically closing lips but the nature of the automatic closing mechanism is not disclosed.
At the time of Velazquez' work it was generally believed by those skilled in the art that effective circulation control forces could be obtained only if the air were discharged at speeds approaching sonic, through extremely narrow slots of widths on the order of 0.5 mm. To produce the very high speed air flow than believed necessary was very inefficient and wasteful of power. In addition, there was a possibility that the extremely narrow slots might become clogged or their critical dimensions might become altered.
In his patent, Velazquez states that his invention comprehends the use of a high-pressure-ratio supersonic fan such as is currently employed in modern turbofan engines. That Velazquez contemplated the use of a high velocity airstream discharged through the circulation control slots is evidenced further by his use of a diffuser in conjunction with the high-pressure-ratio supersonic fan.
Further, the vanes which Velazquez provides at the tail are adapted for producing both forward thrust and antitorque forces, and therefore are less efficient in producing the antitorque forces than if they had been designed solely for that purpose. Thus, although Velazquez combines the jet thruster with circulation control, his antitorque system is highly inefficient because it was based on the then-prevailing understanding that effective circulation could only be obtained by the use of a very high velocity discharge through an extremely narrow slot.